The present invention relates to the non-cryogenic separation of gas mixtures. The invention provides an improved method and system for separating a feed gas into components. The invention is especially suited to separation of a feed gas which contains condensable hydrocarbons.
It has been known to use a polymeric membrane to separate air into components. Various polymers have the property that they allow different gases to flow through, or permeate, the membrane, at different rates. A polymer used in air separation, for example, will pass oxygen and nitrogen at different rates. The gas that preferentially flows through the membrane wall is called the “permeate” gas, and the gas that tends not to flow through the membrane is called the “non-permeate” or “retentate” gas. The selectivity of the membrane is a measure of the degree to which the membrane allows one component, but not the other, to pass through.
A membrane-based gas separation system has the inherent advantage that the system does not require the transportation, storage, and handling of cryogenic liquids. Also, a membrane system requires relatively little energy. The membrane itself has no moving parts; the only moving part in the overall membrane system is usually the compressor which provides the gas to be fed to the membrane.
A gas separation membrane unit is typically provided in the form of a module containing a large number of small, hollow fibers made of the selected polymeric membrane material. The module is generally cylindrical, and terminates in a pair of tubesheets which anchor the hollow fibers. The tubesheets are impervious to gas. The fibers are mounted so as to extend through the tubesheets, so that gas flowing through the interior of the fibers (known in the art as the bore side) can effectively bypass the tubesheets. But gas flowing in the region external to the fibers (known as the shell side) cannot pass through the tubesheets.
In operation, a gas is introduced into a membrane module, the gas being directed to flow through the bore side of the fibers. One component of the gas permeates through the fiber walls, and emerges on the shell side of the fibers, while the other, non-permeate, component tends to flow straight through the bores of the fibers. The non-permeate component comprises a product stream that emerges from the bore sides of the fibers at the outlet end of the module.
Alternatively, the gas can be introduced from the shell side of the module. In this case, the permeate is withdrawn from the bore side, and the non-permeate is taken from the shell side.
An example of a membrane-based air separation system is given in U.S. Pat. No. 4,881,953, the disclosure of which is incorporated by reference herein.
Other examples of fiber membrane modules are given in U.S. Pat. Nos. 7,497,894, 7,517,388, 7,578,871, and 7,662,333, the disclosures of which are all hereby incorporated by reference.
Most gas streams, whether they originate from wells in the field, or whether they are taken from industrial processes, contain various impurities which must be removed, or minimized, if the gas stream is to be commercially useful. Typically, such impurities include water vapor, particulates, condensable hydrocarbon gases, and other less desirable gases.
Water vapor is usually removed with a vapor trap, while condensable hydrocarbon gases are removed by a carbon bed filter. Less desirable gases are often removed through the use of a selective membrane, or other gas separation method such as pressure swing adsorption or a cryogenic process.
Many polymer membranes become degraded in the presence of liquid water or water vapor. In such cases, the air directed into the membrane must be substantially free of water. For this reason, it is common to provide some form of dehydration unit which treats the gas before it flows through the membrane. Polymers have been developed which separate water vapor from a gas. An example of such a polymer is given in U.S. Pat. No. 7,294,174, the disclosure of which is incorporated by reference herein.
A polymer membrane may also be degraded by oil particulates and oil vapor, which may leak from the compressor.
In addition to a dehydration module and a carbon bed, one may provide heaters, moisture traps, and/or filters between the compressor and the membrane unit, as needed.
Some membranes are degraded by the presence of condensable hydrocarbons, especially those of high molecular weight. Such hydrocarbons, if present in a feed stream, may condense in the membrane, and will thus reduce the processing rate with regard to the incondensable components. The condensation thus reduces the overall efficiency of the gas separation process. If such condensable hydrocarbons are removed from the feed stream, the useful life and stability of the membrane can be readily increased.
Systems of the prior art have addressed the problems caused by the presence of volatile higher hydrocarbons by using process stream chillers and/or carbon beds, positioned upstream of the gas separation unit. A chiller causes the hydrocarbon to condense, so that the hydrocarbon can be conveniently removed as a liquid, before the feed gas flows into the membrane module.
The temperature to which the gas may be chilled is effectively limited, however, because it is necessary to avoid freezing the components of the feed gas, such as hydrocarbons, water vapor, and/or hydrates which could cause plugging of the process lines. Thus, chillers are of limited utility in reducing the concentration of hydrocarbons in the gas.
Carbon beds are useful in pre-treating a gas mixture, but such beds can quickly become filled with hydrocarbons, and the beds are difficult to regenerate. Such regeneration typically requires extremely high temperatures, and the regeneration process may take considerable time, further reducing the efficiency of the process.
Other examples of prior art devices for removal of hydrocarbons from gas streams are shown in U.S. Pat. Nos. 4,553,983, 5,772,734, and 6,352,575.
One application in which the present invention is especially useful, is the production of natural gas (methane). The gas taken from the well typically includes water vapor, carbon dioxide, and some heavier hydrocarbons, in addition to the methane. The object is to remove everything but the methane, or to produce a gas which is mainly methane with some additional hydrocarbons. However, no single membrane will work well under these conditions. A membrane which may work well in removing carbon dioxide or water may degrade quickly in the presence of condensable hydrocarbons.
The present invention solves the problem caused by differences in the properties of different membrane materials. The invention provides a commercially workable system and method for membrane-based separation of a gas, making possible the efficient removal of various impurities. The invention is not limited to use in the example given above, but can be used in other gas-separation applications.